What does P represent in the equation ?

⚑ Flag

The probability of the dominant allele in the population.  Remember that can appear in the homozygous dominant phenotypes(p²) or the heterozygous (pq).  So p² + 2pq is the probablity of the dominant phenotype in the population.  Since q² is the recessive phenotype probablity, take the square root  of that frequency and you'll have q. Subtract this from 1 to find p.